feat(server/metrics): add documentation around the new metrics endpoint

docs/User Guide/User Guide/Advanced Usage/Configuration (config.ini or e.md

@@ -27,4 +27,4 @@ The code will:
 1.  First load the `config.ini` file as before
 2.  Then scan all environment variables for ones starting with `TRILIUM_`
 3.  Parse these variables into section/key pairs
-4.  Merge them with the config from the file, with environment variables taking precedence
+4.  Merge them with the config from the file, with environment variables taking precedence

docs/User Guide/User Guide/Advanced Usage/Metrics.md

@@ -0,0 +1,110 @@
+# **Trilium Metrics API**
+
+The Trilium metrics API provides comprehensive monitoring data about your Trilium instance, designed for external monitoring systems like Prometheus.
+
+## **Endpoint**
+
+*   **URL**: `/etapi/metrics`
+*   **Method**: `GET`
+*   **Authentication**: ETAPI token required
+*   **Default Format**: Prometheus text format
+
+## **Authentication**
+
+You need an ETAPI token to access the metrics endpoint. Get one by:
+
+```
+# Get an ETAPI token
+curl -X POST http://localhost:8080/etapi/auth/login \
+  -H "Content-Type: application/json" \
+  -d '{"password": "your_password"}'
+
+```
+
+## **Usage**
+
+### **Prometheus Format (Default)**
+
+```
+curl -H "Authorization: YOUR_ETAPI_TOKEN" \
+  http://localhost:8080/etapi/metrics
+
+```
+
+Returns metrics in Prometheus text format:
+
+```
+# HELP trilium_info Trilium instance information
+# TYPE trilium_info gauge
+trilium_info{version="0.91.6",db_version="231",node_version="v18.17.0"} 1 1701432000
+
+# HELP trilium_notes_total Total number of notes including deleted
+# TYPE trilium_notes_total gauge
+trilium_notes_total 1234 1701432000
+
+```
+
+### **JSON Format**
+
+```
+curl -H "Authorization: YOUR_ETAPI_TOKEN" \
+  "http://localhost:8080/etapi/metrics?format=json"
+
+```
+
+Returns detailed metrics in JSON format for debugging or custom integrations.
+
+## **Available Metrics**
+
+### **Instance Information**
+
+*   `trilium_info` - Version and build information with labels
+
+### **Database Metrics**
+
+*   `trilium_notes_total` - Total notes (including deleted)
+*   `trilium_notes_deleted` - Number of deleted notes
+*   `trilium_notes_active` - Number of active notes
+*   `trilium_notes_protected` - Number of protected notes
+*   `trilium_attachments_total` - Total attachments
+*   `trilium_attachments_active` - Active attachments
+*   `trilium_revisions_total` - Total note revisions
+*   `trilium_branches_total` - Active branches
+*   `trilium_attributes_total` - Active attributes
+*   `trilium_blobs_total` - Total blob records
+*   `trilium_etapi_tokens_total` - Active ETAPI tokens
+*   `trilium_embeddings_total` - Note embeddings (if available)
+
+### **Categorized Metrics**
+
+*   `trilium_notes_by_type{type="text|code|image|file"}` - Notes by type
+*   `trilium_attachments_by_type{mime_type="..."}` - Attachments by MIME type
+
+### **Statistics**
+
+*   `trilium_database_size_bytes` - Database size in bytes
+*   `trilium_oldest_note_timestamp` - Timestamp of oldest note
+*   `trilium_newest_note_timestamp` - Timestamp of newest note
+*   `trilium_last_modified_timestamp` - Last modification timestamp
+
+## **Prometheus Configuration**
+
+Add to your `prometheus.yml`:
+
+```
+scrape_configs:
+  - job_name: 'trilium'
+    static_configs:
+      - targets: ['localhost:8080']
+    metrics_path: '/etapi/metrics'
+    headers:
+      Authorization: 'YOUR_ETAPI_TOKEN'
+    scrape_interval: 30s
+
+```
+
+## **Error Responses**
+
+*   `400` - Invalid format parameter
+*   `401` - Missing or invalid ETAPI token
+*   `500` - Internal server error